In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
Various types of inks are used in commercial lithographic printing presses, and three press components make repeated contact with ink: the printing member itself, inking rollers, and the blanket cylinder. As a result, these components require cleaning between print jobs or during maintenance. The printing member is cleaned as it is readied for use, e.g., by exposure to fountain solution, while the inking rollers and offset blanket are typically cleaned with a press cleaner (often referred to as “press wash,” or “roller and blanket wash”). Conventional press-cleaning compositions contain petroleum-based solvents such as naphtha, mineral spirits, toluene and/or xylene. Such solvents release volatile organic compounds, or “VOCs”—i.e., carbon-containing materials that evaporate into the air. VOCs are environmentally deleterious, contributing to the formation of smog and posing potential toxicity hazards.
In addition, solvents having high VOC contents (50 to 100%) may often penetrate the offset blanket and cause it to swell, increasing its thickness and potentially leading to changes in impression pressure that create printing defects. Unfortunately, high-VOC compositions are particularly effective in removing ink.
Low-VOC press-cleaning agents with acceptable performance have been introduced, but these tend to work best with heat-set or cold-set inks. Inks curable by exposure to ultraviolet (UV) radiation are particularly difficult to clean without high-VOC solvents. UV-curable inks are considered “100% solid systems” in that they contain only pigment and acrylate monomers; although they are not dry (having, instead, a paste-like viscosity), they do not contain solvents. The “diluent monomers” and “resin prepolymers/oligomers” (which are either epoxy acrylates, polyester acrylates or polyurethane acrylates) found in these inks are not used in typical heat-set or cold-set inks, and make UV-curable inks difficult to clean with typical low-VOC cleaning compositions.